Sulfur compounds (and especially hydrogen sulfide) are typically removed from a waste gas prior to venting into the atmosphere using one or more Claus plant sulfur recovery units that include a thermal stage followed in series by one or more catalytic stages. While sulfur recovery using a Claus plant is conceptually simple and well known in the art, effective operation of a Claus plant is not trivial due to numerous variable parameters. Among other factors, the chemical composition (e.g., content and relative proportions of hydrogen sulfide, carbon dioxide, and water) of the feed stream into the Claus plant may change considerably dependent on the type of facility and processes used upstream of the Claus plant. Therefore, based on the specific stoichiometric requirements of the Claus reaction, stringent control of oxygen quantities for the thermal stage is critical for effective operation of a Claus plant.
In most currently known configurations with a single thermal stage, constant chemical composition of the waste gas feed is assumed for control of the amount of oxygen needed in the thermal stage. To accommodate small variations in hydrogen sulfide concentration in the feed gases, and to account for inaccuracies in the thermal stage control instrumentation, it is common practice to install feedback control instrumentation in such plants to make fine adjustments to the thermal stage air demand logic. The feedback logic in such systems typically involves gas analysis of the molar ratio of hydrogen sulfide to sulfur dioxide in the tail gas leaving the final catalytic stage. A control signal from a tail gas analyzer is then used to make small changes to the quantity of air or other oxygen-containing gas that is delivered to the thermal stage to achieve the desired ratio of hydrogen sulfide to sulfur dioxide in the effluent stream. A typical example for such configuration is described in U.S. Pat. No. 4,100,266 where flow of an oxygen-containing gas is regulated using a controller that operated on the basis of measured oxygen concentration in the oxygen-containing gas and measured concentration of various components in the tail gas and vented gas stream. Similarly, RE028864 describes a system in which a control signal to regulate flow of oxygen or oxygen containing gas is generated from (a) measured concentrations of hydrogen sulfide and oxygen at the inlet of the thermal stage and a corrective value, and (b) measured concentrations of components in the tail gas.
In further known configurations (e.g., WO 2006/005155, or U.S. Pat. No. 3,026,184), process control is achieved using measurements downstream of both the thermal stage and the catalytic stage to form a combined control signal that is then used to directly regulate the flow of the oxygen-containing gas to the thermal stage. Combined control signals allow for increased fine-tuning of oxygen flow based on two process points, however, will typically not allow differentiation between imbalances at the two process points.
Alternatively, temperature control of the thermal stage may be employed to optimize the overall performance of a Claus plant as described in U.S. Pat. No. 4,543,245, and in yet another known approach, oxygen feed to the thermal stage can be controlled by calibrating a hydrocarbon-representative response signal (rather than a hydrogen sulfide representative response signal) that is responsive to the ratio of hydrogen sulfide/sulfur dioxide in the Claus plant tail gas as described in U.S. Pat. No. 4,836,999.
While such known tail gas control circuits tend to operate satisfactorily under many circumstances, various difficulties remain, especially in relatively large Claus plants that need to process very large quantities of sour feeds. Such plants often include several thermal stages operating in parallel followed by one or more catalytic stages operating in series. Unfortunately, such known configurations with parallel thermal stages present problems with feedback control from the tail gas analyzer (typically measuring ratio of hydrogen sulfide to sulfur dioxide). For example, the desired tail gas ratio may not be achieved where one of the thermal stages operates with too much air or oxygen while the other thermal stage(s) operate(s) with the correct amount or too little air. Since in such plants the tail gas analyzer is positioned downstream of the common catalytic stage, the downstream analyzer is insensitive to differences between the independently operating thermal stages. As such, the analyzer's feedback control signal will take the correct action for one of the thermal stages, but an incorrect action for the other(s), potentially intensifying the problem. Thus, control of the Claus plant may continually swing from tail gas hydrogen sulfide to sulfur dioxide ratios that are too high, to ratios that are too low.
To circumvent at least some of the problems associated with plants having multiple parallel thermal stages, a Claus plant configuration can be implemented in which oxygen flow control to the additional thermal stage is achieved by measuring the flow rate of combustible gas into the additional thermal stage and the ratio of hydrogen sulfide to sulfur dioxide in the sulfur depleted gas stream from the additional thermal stage as described in U.S. Pat. No. 6,287,535. While such configurations and methods advantageously allow for significantly increased throughput of combustible acid gas, several problems nevertheless remain. Once more, any deviation of a desired ratio between hydrogen sulfide to sulfur dioxide in the tail gas can not be traced back to a particular thermal stage that produced or precipitated the deviation.
Therefore, while numerous methods of operational control for Claus plants are known in the art, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved configurations and methods for control in Claus plants.